Infection of mice with Sindbis virus (SINV) provides a model for examining the role of the immune response to alphavirus infection of the central nervous system (CNS). Interferon-gamma (IFN-γ) is an important component of this response, and we show that SINV-infected differentiated neurons respond to IFN-γ in vitro by induction of antiviral genes and suppression of virus replication.To determine the in vivo effects of IFN-γ on SINV clearance and T cell responses, C57BL/6 mice lacking IFN-γ or IFN-γ receptor-1 were compared to wild-type (WT) mice after intracranial SINV infection. In WT mice, IFN-γ was first produced in the CNS by natural killer cells and then by CD4 + and CD8 + T cells. Mice with impaired IFN-γ signaling initiated clearance of viral RNA earlier than WT mice associated with CNS entry of more granzyme B-producing CD8 + T cells. However, these mice established fewer CD8 + tissue-resident memory T (T RM ) cells and were more likely to experience reactivation of viral RNA synthesis late after infection. Therefore, IFN-γ suppresses the local development of granzyme B-expressing CD8 + T cells and slows viral RNA clearance but promotes CD8 + T RM cell establishment.Viruses 2020, 12, 113 2 of 25 virus is no longer recoverable, but declining levels of viral RNA are still readily detectable. Finally, in Phase 3, from 60 days through at least a year and presumably for the life of the animal, viral RNA persists at a low level [10,11].The immune response to alphavirus infection in the CNS presents a double-edged sword: while local production of antibody and IFN-γ clears infectious virus [6-9], T cell-mediated inflammation is responsible for many of the pathological changes and much of the neurological damage produced [12][13][14]. CD8 + T cells participate in clearance of viral RNA because CD8 and B2m knockout mice clear viral RNA more slowly from the brains and spinal cords than wild-type (WT) mice [3].IFN-γ, an important product of natural killer (NK) cells and CD4 + and CD8 + T cells, exerts its antiviral effects by inducing IFN-stimulated genes (ISGs), but also by modulating the immune response to infection. The IFN-γ receptor is a heterotetramer of ligand-binding IFN-γR1 and signaling IFN-γR2 expressed on many cell types, including neurons [15]. IFN-γ binding to its receptor triggers a Jak/Stat signaling pathway that can induce expression of over 200 ISGs, some of which have direct antiviral activity, while others modulate the immune response [15][16][17]. IFN-γ is particularly important for clearance of infectious virus from spinal cord neurons [7]. IFN-γ-induced immunomodulatory effects include immune cell activation, trafficking, and differentiation, as well as more direct intracellular antiviral activities [18].IFN-γ is detectable in the CNS within 3 days after infection, peaks at 7 days and becomes undetectable by 10-14 days, although IFN-γ mRNA remains elevated for months [9]. Previous studies have shown that mice with impaired IFN-γ signaling produce lower levels of inflammatory cytokines and chemo...